1. Field of the Invention
This invention relates to the registration of artwork panels in the manufacture of printed circuit boards.
2. Discussion of the Related Art
Conventionally, an artwork panel is used for the preparation of an individual circuitry layer of a printed circuit board. As it is essential to attempt to obtain precise relative layer-to-layer positioning of the individual circuitry layers, the artwork panels for the layers need to be punched-out to provide registration holes which subject to operation error, are disposed in precisely predetermined locations relative to circuitry patterns on the artwork panels. In order to position the artwork panels preparatory to the punching operations, a support surface is provided directed towards which are overhead lights and at least two downwardly directed video cameras. A pre-punched panel is first placed upon the support surface and is adjusted in position until pre-punched areas of that panel are disposed in desired punching positions required for the artwork panels. The predetermined location of the pre-punched panel is obtained when punches of the apparatus are locatable simultaneously through each of the pre-punched holes in the pre-punched panel. With light reflected from the upper surface of the pre-punched panel, the cameras, movable horizontally in two directions normal to each other, are adjusted until they are vertically aligned above precisely positioned circular target holes provided upon the pre-punched panel. It is intended that the cameras will be vertically aligned above the target holes when camera images of the holes are positioned exactly symmetrically within video generated shapes upon video screens connected to the video cameras. The symmetrical positioning is judged visually by an operator who, when the video cameras are considered to be precisely in position to provide the desired symmetry, locks the video cameras in these positions which the cameras must maintain during punching of the artwork panels which is to follow.
The pre-punched panel is then removed and a first artwork panel to be punched is located upon the support surface. Each artwork panel has circular target patterns corresponding in position to the target holes in the pre-punched panel. The first artwork panel is moved upon the support surface in horizontal directions until its target patterns lie in required positions vertically beneath the cameras and in which the camera images of the patterns are judged visually to be positioned symmetrically within the video generated shapes upon the video screens. The first artwork panel is then considered to be positioned correctly for punching out the artwork panel in the predetermined locations. The punching operation then follows. The first artwork panel is then removed and the same judgmental location and following punching procedure takes place sequentially for all other artwork panels of the printed circuit board.
A problem which is inherent with the above procedure is that of quality control. As indicated above, the positioning of the pre-punched panel and of the artwork panels is judged visually. It has been shown that with visual assessment, the error in positioning each target relative to its exact desired position, both upon the pre-punched panel and upon the artwork panels may amount to .+-.0.003 inches. When it is considered that the error in positioning the pre-punched panel and any of the artwork panels may be in opposite directions from the exact desired position, then the error could be accumulative from artwork panel to artwork panel or there could be a resultant error between panels dependent upon an error in positioning of one panel in a direction at an angle to the directional error of another panel.
The above problem is augmented by changes in temperature and humidity of the artwork panels. Changes in temperature and humidity result in shrinkage or expansion of panels thereby changing the distance apart of their circular target patterns. As a result, it may be impossible to simultaneously align target patterns of any particular panel precisely with both video generated shapes on the video screens. The positioning of an artwork panel prior to punching then becomes more open to conjecture thereby enabling positioning errors to increase. A further problem is caused by the fact that as the cameras need to be adjusted in position initially over the pre-punched panel along both the `x` and `y` axes, then it is impossible to mount the cameras rigidly in position. Hence the cameras are subjected to slight vibrations which tend to change the positions of images upon the video screens, a positional change on the screens of course being magnified.